1. Field of the Invention
This invention relates generally to lasers and, more particularly, to aging of tunable semiconductor lasers.
2. Discussion of the Related Art
Recent work has concentrated on producing laser transmitters for use in dense wavelength division multiplexed (DWDM) optical networks. In a DWDM network, data signals are transmitted in a set of adjacent and spectrally narrow bands. Each band functions as a separate communications channel. For such a channel structure, a transmitter must produce a spectrally narrow output that injects optical energy into a single band. Distributed Bragg reflector (DBR) lasers are strong candidates as transmitters for DWDM networks, because DBR lasers have spectrally narrow outputs.
A tunable DBR laser has an output wavelength that depends on the value of a tuning current. The dependence may be shown by an operating characteristic. FIG. 1 shows an initial operating characteristic 10 for one DBR laser. The operating characteristic 10 has a series of steps 11-20. A DBR laser operates in a single mode by staying on one of the steps when functioning as a DWDM transmitter. The output wavelength remains fairly constant while the laser remains on one step. Use physically changes the DBR laser, and the physical changes alter the locations of the steps 11-20 to produce new operating characteristic 22. On the new characteristic 22, an initial value of the tuning current may produce a new output wavelength. Thus, aging can induce drift of the laser""s output wavelength.
Age-induced wavelength drift has serious consequences for DBR lasers that operate as transmitters in DWDM networks. Wavelength drift can cause the laser to hop to a new mode on an operating characteristic. The new mode may have a wavelength that corresponds to a different channel of the DWDM network. Mode hopping can be avoided through closed-loop wavelength feedback, which adjusts the value of the tuning current in response to detecting the onset of a mode hop. Additional closed-loop wavelength feedback can also keep side-band emission levels low, by maintaining operation of the DBR laser near a center of a step. Nevertheless, closed-loop wavelength feedback cannot compensate for changes in the values of tuning currents of other modes, i.e., steps at which the DBR laser is not presently operating. Changes to the value of tuning currents for those modes result in uncertainties on how to change the tuning current to move to a new step on the operating characteristic, e.g., to change the transmission channel in a DWDM network.
In one embodiment, the invention features a process for evaluating an aging property of a distributed Bragg reflector (DBR) laser. The process includes illuminating a Bragg grating of the distributed Bragg reflector (DBR) laser with light while the DBR laser is both supplied a tuning current and not lasing. The process also includes performing an action to the DBR laser responsive to a wavelength of a Bragg peak in a portion of the light reflected by the Bragg grating and a value of the tuning current supplied during the illuminating.
In another embodiment, the invention features a process for operating a wavelength-tunable DBR laser. The process includes operating the DBR laser at a first output wavelength, measuring a value of a tuning current causing the DBR laser to operate at the first output wavelength, and calculating a new value of the tuning current. The act of calculating is based in part on the measured value of the tuning current. The new value is capable of operating the DBR laser at a second output wavelength.
In another embodiment, the invention features an apparatus including a wavelength-tunable DBR laser and a controller coupled to apply a tuning current to an electrical terminal of the DBR laser. The controller is capable of applying a new value of the tuning current to the terminal to cause the DBR laser to jump to a new operating mode and is configured to determine the new value based in part on a measured pre-jump value of the tuning current.
In another embodiment, the invention features a system capable of determining age-induced changes to an output wavelength of a DBR laser. The system includes the DBR laser, a spectrum analyzer positioned to receive light reflected by a tunable Bragg grating of the laser, and a processor coupled to receive data on a reflection spectra of the Bragg grating from the spectrum analyzer. The processor also receives data values of tuning currents associated with the reflection spectra and is configured to determine a functional relation between Bragg peak wavelengths and the values of the tuning currents from the data.